A humidity measurement is very important in many fields such as environment, food, agriculture, medical care, automobile, textile, semiconductor technology, bio technology, and the like. To this end, various humidity measurement technologies using impedance, capacitance, optical method, surface acoustic wave, etc. have been developed. In the humidity measurement, the dew-point measurement has been generally used as a standard method and the dew-point sensor in a chilled mirror type has been currently used as a standard calibrator in many laboratories.
In fields such as semiconductor, display, ultrapure gas manufacture, etc., a plurality of processes has been performed in very low pressure or vacuum environment. In the case of a semiconductor process, although there is an extremely small amount of moisture in a portion of remained gas during the process, the moisture has a large effect on physical properties of a metal and a semiconductor thin film. Further, since an extremely small amount of moisture causes the same defects even in the case of the display process, a need exists for a technology capable of measuring an extremely small amount of moisture such as ppm and ppb within a minimum space.
Therefore, a need for a technology capable of accurately measuring and analyzing the dew point in a low humidity environment has been increased. A currently commercialized dew-point measurement apparatus can measure the dew point up to −90° C., but is difficult to accurately measure supercooled dew point due to low accuracy in low temperature.
As one example, in the case of the dew-point measurement apparatus in the currently generally used chilled mirror type, since it measures a formation of the dew point by means of the optical method, the measurement apparatus can perform the dew-point measurement only the case where a dew of 3 μg/cm2 or more is produced. Also, when measuring the dew point in a low temperature between 0° C. and −40° C., liquid dew may be produced, but solid frost may be produced. However, it is not easy to distinguish them. The temperature of the dew point (or frost point) measured according to whether water drop produced in 0° C. or less is the liquid dew or the solid frost has a large error (about 4° C.) so that the accurate measurement may not be performed. Many methods to remove such an error have been studied; however, many solutions have been currently required.